Identify String Network and export its style accurately

[LHuang2019]

Currently the CyGraphSpace fails to export network that is imported from StringApp accurately. The exported network doesn't capture the correctly background color, background image, and the "glass style."

To solve this issue, the user needs to do the following:

• Correctly identify if the network is a String Network that is imported from StringApp.
• Correct mapping of background color in the exported style json.
• Correct mapping of background image in the exported style json.
• Applies the "glass style" that StringApp has for each of its nodes.

There are multiple ways to approach this problem:

• Let CyGraphSpace to manually examined the exported json to see if the name/database is related to StringApp.
• Modify StringApp code base for correct mapping of background color to nodes.
• Let CyGraphSpace to manually edit the exported json for correct mapping of background image to nodes.
• Create SVG image that represents the "glass style" and apply the image using CyGraphSpace.

[LHuang2019]

The pull request #10 allows CyGraphSpace to identify if a network is a String Network that is imported from StringApp.