PlatformIO Library Manger manifest file for GSM

* This library in Web Registry: http://platformio.ikravets.com/#!/lib/show/51
* Specification: [PlatformIO Library Manager](http://docs.platformio.ikravets.com/en/latest/librarymanager/index.html)
* Integration: [IDE Integration](http://docs.platformio.ikravets.com/en/latest/ide.html)

`library.json` is a library manifest file which can be used with different tools/managers to organise embedded libraries. Currently, only [PlatformIO](http://platformio.ikravets.com) uses it.  How does this look from CLI:

```bash
$ platformio lib search "plotly gsm"
# Found ... libraries:
#
# [ ID  ] Name             Compatibility         "Authors": Description
# -------------------------------------------------------------------------------------
# [ 51  ] Plotly-GSM    arduino, atmelavr     "Plotly": Arduino library for real-time logging and streaming data to online plotly graphs

$ platformio lib install 51
# Installing library [ 51 ]:
# Downloading  [####################################]  100%
# Unpacking  [####################################]  100%
# The library #51 'Plotly-GSM' has been successfully installed!

$ platformio lib show 51
# Plotly-GSM
# -------------
# Authors: Plotly <feedback@plot.ly> https://github.com/plotly
# Keywords: logging, data, streaming, gsm
# Frameworks: arduino
# Platforms: atmelavr
# Version: aa5973b043
#
# Arduino library for real-time logging and streaming data to online plotly graphs

$ platformio lib update
# Updating Plotly-GSM  library:
# Versions: Current=***, Latest=***       [Up-to-date]
```