making magic number-based detection of PDFs encoding-friendly, with tests

Magic number-based PDF-detection is vulnerable to encoding issues; forcing the first line to be read in binary mode should work across encodings (as the "%PDF" marker will be present using ASCII-7 characters in any case).  Also, removing the "end of line" anchor from the PDF version regex to avoid issues with non-printing characters in certain cases.

Added tests, details:
- extracted PDF detection from ensure_pdfs method for more direct testing
- new unit tests verify that PDFs without the ".pdf" extensions are detected as PDFs
  - samples are included under "test/fixtures/without_pdf_extension"
  - add further PDFs (without extensions) and they will be automatically included in tests
  - PDFs generated by Adobe InDesign previously raised encoding errors; samples (for each recent PDF version) are included
- new unit tests verify that files with the ".pdf" extensions are always detected as PDFs (regardless of file contents)
  - samples are included under "test/fixtures/with_pdf_extension"
  - samples file names reflect actual contents
  - not sure that this is the correct behavior, but not looking to change current approach
Merge pull request #108 from jonoterc/fix_pdf_detection

making magic number-based detection of PDFs encoding-friendly, with tests
Fix the tests for Ruby2.1
Update merged test to minitest
Bump docsplit version for release.
Windows:  "%d" is always escaped to "\%d".

Unders Windows, "%d" is always escaped to "\%d". It makes "extract_pages" fail because of below exception.

Docsplit::ExtractionFailed: Unhandled Java Exception in create_output():
java.io.FileNotFoundException: ./abc_\1.pdf: The system cannot find the path specified.
Merge pull request #118 from ypxing/master

Windows:  "%d" is always escaped to "\%d".
Fixes bug on empty result for command "#{office_executable} -h 2>#{null}"

On Debian Jessie with LibreOffice 4.3.2, this line fails because the command returns nothing.
Merge branch 'patch-1'

Conflicts:
	Rakefile
	docsplit.gemspec
	lib/docsplit/transparent_pdfs.rb
Bumps to 0.7.7