Support for uploading resources to FHIR server

[deepankarb]

Is your feature request related to a problem? Please describe.
The SDK supports downloading Resources from a FHIR server and making local modifications. There's no way to upload the modifications back to the server.

Describe the solution you'd like
Upload the changes back to the server in a FHIR-compliant way.

Describe alternatives you've considered
This is a core feature.

Future work
Add support for:

• Resource Conflict resolution
• Preventing multiple process from uploading at the same time [Disable simultaneous upload from multiple processes #103 ]

Also see #71 for download sync implementation.