Introduced ChecksumType: an enum for all supported checksum types.
(Currently MD5 and SHA-1.)
Introduced Checksum: a class that ties together a checksum type and a
value of that type to be checked.
Introduced DownloadItem: a class that ties together an URL and,
optionally, a Checksum instance (or None).
DownloadCenter now takes a list of DownloadItems.
Merge pull request #45 from Tinche/checksums

Introduced ChecksumType: an enum for all supported checksum types.
We allow 120 characters, fix some style changes

Previous merge content in addition to the feature some style changes,
getting the initial formatting back.
New android studio doesn't ship a sdk anymore

So we don't add it to user path
Match the fake asset sha1 in medium tests
Add tests for sha1 matching
releasing package ubuntu-developer-tools-center version 0.1.1-0ubuntu1
Rename create_launcher to post_install

* We are doing other logical things into it like adding directories to PATH
  and soon, adding any env variable as well
Just take the first result matching our parser
Add remove_framework_envs_from_user facility

This enables to remove what a framework is setting in user .profile file.
Wire removing frameworks to remove framework env

* Removing a framework remove all tagged content for this framework
* Reinstalling a framework will remove previously tagged content and will then
  only use the new paths/envs.
Replace add_to_user_path by add_env_to_user

* add_env_to_user enables to add any arbitrary environment to the user
* change .bashrc -> .profile to not only support bash
* Support reinstalling on other env if the framework tag was already there
* If keep is set to True, we keep the previous env variable and don't overwrite
  it
Adapt android env with the new facility

Replace the PATH for adding android adb tools if needed using the new facility
Tests for changing the user env

* Adapt existing tests for changing the user env
* Add more tests for corner cases and when removing user env on framework
  removal
Change tools add_env_to_user API

* Accept simultaneous multiple envs additions in add_env_to_user
Adapt tests to new API
Enhance existing tests

Better check what we actually want to check
Test appending multiple envs for the same fw
Export variables that are not PATH
Adapt tests for the export feature