switch(gettype) instead of switch(true) + is_*()

The semantics is exactly the same but the code is easier to understand,
faster to execute and easier to statically analyzed.
Merge branch 'master' of https://github.com/atoum/atoum into asserter_gettype

Conflicts:
	classes/asserter.php
Merge pull request #190 from Hywan/asserter_gettype

switch(gettype) instead of switch(true) + is_*(), thanks to @Hywan.