Changed expected duplication size in DetectorTest

The default detection strategy currently does not count the number of tokens
in a duplication correctly; it neglects the last ($minTokens - 2) tokens. This
commit changes DetectorTest to expect the detector to find the duplication in
Math.php to its full extent.
Added three tests to DetectorTest

The test currently present in testDetectingSimpleClonesWorks is hard to verify
manually, since the number of tokens cannot be counted easily.

This commit adds two identical files, a.php and b.php, in which the number of
(non-ignored) tokens can be conveniently counted. Since phpcpd (currently)
ignores the opening tag, whitespace, comments and the '=', '+' and ';' the
duplication between these files spans exactly 60 tokens and 20 lines.

Using these two files, three tests cases are added to DetectorTest:
 * One that verifies the duplication is found when minLines = 20 and
   minTokens = 60
 * One that verifies the duplication is ignored when minLines = 20 and
   minTokens = 61
 * One that verifies the duplication is ignored when minLines = 21 and
   minTokens = 60

Since these tests can easily be checked by hand, they can be used to verify
that the detector reports duplications with the correct token and line count,
and ignores duplications that are smaller than the configured thresholds.
Fixed incorrect token count in DefaultStrategy

DefaultStrategy finds duplications using a 'sliding window' of $minTokens
tokens. When it detects that the signature of the tokens in the current window
was seen before it keeps moving its window forward until the signature does not
match an earlier signature anymore (or the end of the file is reached). This
indicates the end of the duplication.

When the end of a duplication is detected, the number of the last duplicated
token was incorrectly calculated. Rather than the start token of the current
window it should be the last token of the previous window, since that window
was duplicated in its entirety. The last token of the duplication is therefore
not $tokenNr, but ($tokenNr - 1) + $minTokens - 1: the start token of the
previous window plus the window length minus 1.

Also the loop that 'moved the window forward' was incorrect; it can stop as
soon as there does not fit an entire window of $minTokens tokens in the file
anymore. In that case there can never be a duplication of $minTokens tokens
at the end of the file anymore.
Removed obsolete if-statement in DefaultStrategy

The condition inside this if-statement is now already implicitly present in
the condition of the while-loop beneath it.
Introduced $numLines-variable for readability
Merge pull request #65 from aboks/fix_token_count