doc: added and fixed some copyright boilerplates
cpu: removed NAKED attribute from ISRs

- removed the __attribute__((naked)) from ISRs
- removed ISR_ENTER() and ISR_EXIT() macros

Rationale: Cortex-Mx MCUs save registers R0-R4 automatically
on calling ISRs. The naked attribute tells the compiler not
to save any other registers. This is fine, as long as the
code in the ISR is not nested. If nested, it will use also
R4 and R5, which will then lead to currupted registers on
exit of the ISR. Removing the naked will fix this.
Merge pull request #1888 from OlegHahm/license_cleanup

doc: added and fixed some copyright boilerplates
unittests: add test network device

The reasoning behind this commit is twofold:
1. test the "lower" part (the device part) of netdev
2. provide a well defined testing infrastructure for modules that utilise
   netdev
The actual tests that represent 1. are provided in the following commit
unittests: Provide unittests for net_dev_dummy

* tests if net_dev_dummy is correct and if driver part of net_dev is
  correct
Merge pull request #1731 from authmillenon/netdev-test-dummy

netdev: Provide test network device with netdev interface
Merge pull request #1824 from haukepetersen/fix_cortex_isrs

cpu: removed NAKED attribute from ISRs
added board airfy beacon