Add git http fetcher (smart protocol)

This adds a class to fetch / push git objects through
smart git transfer protocol.
mock file system
implement a test
Include all required CSS dependencies from spark.css, not spark.html
Improve tab view
Merge pull request #244 from ikarienator/TabViewImpr

Improve tab view
add support for context enabled actions
doc and style cleanups
finish up work to implement vim and emacs key bindings
Merge branch 'master' into context-menu
Implemented Alt to open a new tab
fix double click on folder; speed up app launch
Merge pull request #229 from dart-lang/files-mock-2

Files mock 2
Merged
Simplified code. Fixed the merge.
Merge pull request #251 from dart-lang/double-click-folder

fix double click on folder; speed up app launch
Merge pull request #238 from ussuri/single-css-entry

Include all required CSS dependencies from spark.css, not spark.html
Fixed nits
Merged
Fixed merge