... and use sstreams instead of sprintf

This commit fixes the formatting warning in the Mutation classes.

- For mut_type() and sense() calls, we changed the PRId8 format specification
  to PRId32
- For the -1 constants, we explicitly cast them to the right size
  (int8_t or int16_t) to reflect the format specification

Magic command for reference:
find ! -path '*test*' ! -path './libaevol/SFMT-src-1.4/*' \( -name '*.cpp' -o -name '*.h' \) -print0 | xargs -0 sed -i -r 's/\bget_(\w+)\b/\1/g'

Looking forward to honour Google Style guide, the purpose is to rename getters
from get_var() to var(). Many of such renames cause conflicts, though. In this
commit, only the safe names were changed.

grep -rEo '\bget_\w+\(' --include '*.h' --no-filename --exclude-dir tests \
 | sed -e 's/get_//' -e 's/($//' \
 | sort -u \
 | xargs -n 1 -I% sh -c "grep -roq '\b%\b' --include '*.cpp' --include '*.h' --exclude-dir tests || echo %" \
 | xargs -n1 -I% sh -c "find ! -path '*test*' ! -path './libaevol/SFMT-src-1.4/*' \( -name '*.cpp' -o -name '*.h' \) -print0 \
 | xargs -0 sed -i -r 's/\bget_%\(/%\(/g'"

Yes. I know...

http://google-styleguide.googlecode.com/svn/trunk/cppguide.html#Function_Names

Instead of a single class with switches everywhere, mutations are now
handled through a proper polymorphism.

Here is the class hierarchy:
Mutation
  <-- LocalMutation
        <-- PointMutation
        <-- SmallInsertion
        <-- SmallDeletion
  <-- Rearrangement
        <-- Duplication
        <-- Deletion
        <-- Translocation
        <-- Inversion
  <-- HT
        <-- InsertionHT
        <-- ReplacementHT