Adding iOS background upload support

path sent to unlink is now checked for file prefix.

Fix 'Illegal Callback Invocation' Android exception

fix progress and uploadProgress not callback in iOS

Fixes a case where `callback` was called twice when something went wrong in the `cp` operation, leading to a `java.lang.RuntimeException: Illegal callback invocation from native module. This callback type only permits a single invocation from native code.`

This is the same fix [as seen in the original repo](https://github.com/wkh237/react-native-fetch-blob/pull/408).

change android request builder so it wont set Content-Type header to …

(small fix) Fix link to issue which was giving 404

Doc update to include note on upgrading across branches

improvement way to get root SDK versions

Update README.md with tip on project references

Wanted to include a note for folks upgrading from the old branch to the new branch about renaming all product references as this was a problem I came across in #122 (https://github.com/joltup/rn-fetch-blob/issues/122)

Fixed trusty example calling `then` instead of `fetch`

just added a function which acts like a ternary with fallback option. Hence, less cluttered "def" variables  
additionally, changed the SDK values from 23 to 26 as per new changes from react-native and Android

Android Target API Level 26 will be required in August 2018.
https://android-developers.googleblog.com/2017/12/improving-app-security-and-performance.html

And the React Native team is already working on this:
facebook/react-native#18095
facebook/react-native#17741

PS: I am aware of this PR https://github.com/joltup/rn-fetch-blob/pull/128 but first its still targeting old SDK values i.e 23 as fallback and secondly I am not sure of the use of `project` to get the value instead of proper way i.e `rootProject.ext` to get the property value.

Removed output directory check / create. Fixes all file types moving.

Added check for failing to create the output directory

Fixed truncating of progress report values.

Added null and path key check on addCompleteDownload method

Fixed file moving

Fix installation via github instructions

update README.md to inform the handle of cookies

Fix notification value for android download Manager