Add more validation checks

[sappelhoff]

Add more checks in validator.js, put adequate tests into test_validator.js ... and then add these checks to the validateBrainVision function in index.js.

Some ideas:

• [x] check if file size of .eeg, .vmrk, .vhdr is adequately big
• [x] make sure .eeg is binary (e.g., using https://github.com/bevry/istextorbinary)
• [x] estimate filesize of .eeg using SamplingInterval and nChannels
• [x] check if files are readable
• [x] check if all mandatory sections are present, such as [Binary Infos], [Common Infos], ...
• [x] within the mandatory sections are all non-default fields present? Such as SamplingInterval
• [x] check the first line of vhdr: is it the header? --> which version of BrainVision format?
• [x] check that links (file pointers) are relative and do not contain idiosyncratic paths because that will make copying the files troublesome (e.g., not /home/stefan/Desktop/test.vhdr but simply test.vhdr)

the more sensible checks, the merrier.

Better to make them fine-grained, so that meaningful issue messages can be pushed in index.js

[sappelhoff]

distributed between single issues