make: comprehensive list of the boards' filesizes
make: add maketarget to to show the biggest obj files
make: show diffence in buildsizes for two folders
Proposal for common 802.15.4 radio driver API definition.
shell: strings in shell_command_t are const
Add basic travis-ci integration

This PR adds the basic integration for [Travis CI][tci].

The config file `.travis.yml` tells the server to setup the basic build
system, and run `compile_test.py`. If not all  examples and tests could be
built, then the Travis CI integration adds a warning to "merge" button on
the bottom of the Github page.

Of course this integration makes little sense until #1049 is resolved,
because new bugs and old bugs cannot be told apart. Also (of course)
only because everything still builds, a PR still can intruduce a
multitude of runtime errors.

This integrations is meant to work additionally to our Jenkings. Users
can activate the test in their own RIOT forks to run the test before
opening a PR to master.

  [tci]: https://travis-ci.org/
shell: Comment internal members, too

Closes #979.
Merge pull request #1065 from Kijewski/issue-979-2

shell: Comment internal members, too
Merge pull request #925 from rousselk/radio-driver-api-proposal

Proposal for common 802.15.4 radio driver API definition.
tests: added missing chronos to blacklist
Merge pull request #1063 from Kijewski/shell_command_t-fix-const

shell: strings in shell_command_t are const
Merge pull request #1066 from mehlis/test-bloom-bytes-followup

tests: added missing chronos to blacklist
test: thread: blacklist boards
Merge pull request #1062 from mehlis/test-thread-blacklist

test: thread: blacklist boards
Merge pull request #1055 from Kijewski/travis-ci

Add basic travis-ci integration
Merge pull request #1033 from Kijewski/buildsizes

make: comprehensive list of the boards' filesizes
make speed up the build process