[BUILD] do not warn about unused functions

- Wunused-function is a concept with very limited utility and it will
  warn about all files that include a header with static functions but
  does not use _all_ of them.
- for example: ITRAQLabeler.cpp includes MatrixUtils.h and uses the
  function convertOpenMSMatrix2EigenMatrixXd but does not use
  matrixIsIdentityMatrix -> a warning will be emitted about the unused
  matrixIsIdentityMatrix function.
[FIX] removed unused typedefs
[FIX] add static cast (no narrowing conversion)
made SplineSpectrum::Navigator public (not usable otherwise); fix compile error (VS2013) due to missing include; fixed interface(names); fixed intendation
fixed segfault (>=0 issue)
hide default Ctor; add missing tests (no content)
fixed loop vars
error handling for faulty spectra; more cleanup and tests
[FIX,TEST] MRMRTNormalizer_test: Windows compatibility
[FIX] fixed uninitialized pointer warning
Merge pull request #836 from grosenberger/fix/mrmrtnormalizer_test

[FIX,TEST] MRMRTNormalizer_test: Windows compatibility
Merge pull request #838 from hendrikweisser/fix-misc

[FIX] fixed uninitialized pointer warning
Merge pull request #835 from cbielow/fix_splineSpectrum

Fix spline spectrum
Merge pull request #834 from hroest/fix/fixWerror

Fix/fix werror
[FIX] fix pyOpenMS test
Merge pull request #840 from hroest/fix/fixpytest

[FIX] fix pyOpenMS test
[FIX] last fix for Werror on gcc 4.8.1
[BUILD] disable -Werror by default

- configure travis to use -Werror in gcc to check all incoming pull
  requests
- Werror does not seem to work on all configurations reliably (e.g.
  exclusion of system libraries) and changes from one compiler version
  to another make this not a safe option for an initial compile of
  OpenMS
[FIX] fix dereference of iterator

- vector iterator front() is not dereferencable if vector is empty
- thanks to @aiche for finding the error!
[FIX] fixed reading of empty spectra