Makefile: always create directories before writing to them

Simplify rules by removing $(TARGET_C) as explicit target.
Update DoubleMatrix documentation

- Remove misleading comment about not checking the input array in DoubleMatirx(int, int, double...)
- Clarify the copy or view semantics of the various constructors
Optimize matrix constructor to avoid extra array allocation, and other more trivial quick ones like not compiling regexes repeatedly in loops.
Merge pull request #38 from srowen/MatrixConstructorOptimization

Optimize matrix constructor to avoid extra array allocation
Merge pull request #33 from keszybz/fix-parallel-make

Fix Makefile to build properly with -j
Merge pull request #37 from chmp/master

Update DoubleMatrix documentation