Bugfixes, code restructure and merge to master

After a long time releasing to the sdk-8-support branch, this release is moving development back
into master. This means that we're moving the old master branch into a new legacy-sdk-6-7 branch, to
allow those who are heavily invested in the old SDKs to continue using them. As with the old
master, this legacy branch will not see new features. The sdk-8-support branch has been merged to
master, and will no longer be present in the repository.

This release targets a series of issues reported since 0.8.2, in addition to a superficial
change to the cache system: We've taken the caches out of the version_handler module into its own
module, named handle_storage. The handle storage acts as a pure storage module, serving the
version_handler. The version_handler (and it alone) manipulates the handle storage.

New features

• The bootloader now yields TX events after successful transmits, one per channel transmitted on.
• Added a bootloader_verify.py script under nRF51/bootloader/pc-util/, to ease debugging of any
  unintended bootloader behavior.

Fixes

• Fix for Issues #72, #74, #75, #78, #79, #80, #82.
• Fix for example case in Issue #91, still following up on this one.
• CRC false-positives should no longer occur. Symptoms included unused handles getting data and
  erronous payloads getting through to the application.
• DFU signature verification reboot-loophole closed.
• Fixed some erronous DFU packet lengths.
• Restructure of version_handler -> new module handle_storage encapsulating caches.
• Removed Bootloader journaling function, replaced by full check verification.
• Fix GCC linker script for Bootloader.
• Improved documentation for Bootloader usage.