Fixes a bug in sort_by_key algorithm

Values were written over by junk data instead of being sorted by keys.
Moreover, this was not covered by tests.

Closes #104, closes #105.
Use lcov to generate coverage info for coveralls.io
Merge pull request #106 from ddemidov/coveralls-lcov

Use lcov to generate coverage info for coveralls.io
Fix file paths for coveralls.io

Reference #106
No need to define SHOW_KERNELS in build_init

Environment variable works better
Provide is_device_filter metafunction

Checks if the provided type is a device filter.
Use is_device_filter to disambigute Context constructors
Provide alternative Context constructor for user-supplied contexts/queues
Duplicate device in tests if only one is available

This makes sure that multidevice functionality is thoroughly tested.
Revert "Use is_device_filter to disambigute Context constructors"

This reverts commit 6ac56a5551060a0c817333dad886c084dd27214b.
is_device_filter does not work with gcc 4.6
Remove ambiguous version of vex::Context constructor
Change Travis-CI configuration

1. Do not run tests if there was a build failure.
2. Run tests once. Since there are now two queues per CPU on a travis
   host, it is possible to test both branches of kernel cache code in
   one run.
Documentation fix
Extend vector grammar with subscript operation

This allows more natural work with raw pointers.
Merge pull request #107 from ddemidov/pointer_subscript

Extend vector grammar with subscript operation
README updated
Try to make { placement more consistent
Merge remote-tracking branch 'origin/master' into clogs-scan
Fix clogs scan when init is non-zero and there are multiple partitions