js/download: Create a stripped records copy

This strippedRecords variable contains an array of records in which each
of the properties which are blank have been removed. This results in
smaller XML and JSON exports, transforming:

    <record>
        <notes>Different classes of protein Listed, not stated in
        original text which class was analysed      </notes>
    <sequence>http://www.uniprot.org/uniprot/?    query=Alcohol+Dehydrogenase+Equus+caballus+&amp;sort=score</sequence>
        <mw>80000</mw>
        <pubmed></pubmed>
        <organ>Liver</organ>
        <pi>8.51</pi>
        <dataset>1976</dataset>
        <method></method>
        <alt_name>Aldehyde reductase</alt_name>
        <name>Alcohol Dehydrogenase </name>
        <pi_range_min></pi_range_min>
        <sub_mw>41000</sub_mw>
        <species>http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?lvl=0&amp;id=9796</species>
        <citations></citations>
        <created_at>2014-01-09 12:54:52.681239</created_at>
        <temp></temp>
        <pi_range_max></pi_range_max>
        <no_iso>12</no_iso>
        <pi_max></pi_max>
        <ec>1.1.1.1</ec>
        <sub_no>2</sub_no>
        <abstract></abstract>
        <source>Equus caballus (Horse)</source>
        <valid>CU</valid>
        <pi_major></pi_major>
        <available_at>http://localhost:5000/r/6</available_at>
    </record>

into:

    <record>
        <notes>Different classes of protein Listed, not stated in original text which class was analysed  </notes>
        <sequence>http://www.uniprot.org/uniprot/?query=Alcohol+Dehydrogenase+Equus+caballus+&amp;sort=score</sequence>
        <mw>80000</mw>
        <organ>Liver</organ>
        <pi>8.51</pi>
        <dataset>1976</dataset>
        <alt_name>Aldehyde reductase</alt_name>
        <name>Alcohol Dehydrogenase </name>
        <sub_mw>41000</sub_mw>
        <species>http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?lvl=0&amp;id=9796</species>
        <created_at>2014-01-09 12:54:52.681239</created_at>
        <no_iso>12</no_iso>
        <ec>1.1.1.1</ec>
        <sub_no>2</sub_no>
        <source>Equus caballus (Horse)</source>
        <id>6</id>
        <valid>CU</valid>
    </record>

Issue #274.
Merge branch 'issue/274'