Add a feature in `CsvSchema` to allow definition of ordering

[cowtowncoder]

(note: cleaved off of #40)

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Currently the only way to specify ordering of columns with CsvSchema is to either use @JsonPropertyOrder annotation in POJO class, or to manually build schema.
But there is no way to indicate alternate ordering, explicit (by listing possibly partial list of columns) or otherwise (defining a strategy to use for sorting).
But there should be a way: for example, by having method(s) in CsvSchema to allow re-ordering of columns.

[cowtowncoder]

Added CsvSchema.sortedBy() methods, matching tests.

[ShijunK]

It seems the implementation only sorts the columns in headers, not actual values.

I have a test for 2.6.3

   public void testValueReorderByName() throws Exception
    {
        CsvMapper mapper = mapperForCsv();
        CsvSchema schema = mapper.schemaFor(Mixed.class);
        assertEquals(aposToQuotes("['a','b','c','d']"), schema.getColumnDesc());
        schema = schema.sortedBy("b", "c");
        assertEquals(aposToQuotes("['b','c','a','d']"), schema.getColumnDesc());

        Mixed value = new Mixed();
        value.a = 1;
        value.b = 2;
        value.c = 3;
        value.d = 4;

        schema = schema.withHeader();
        String outputString = mapper.writer(schema).writeValueAsString(Arrays.asList(value));
        assertThat(outputString, is(equalTo("b,c,a,d\n2,3,1,4\n")));

        _verifyLinks(schema);
    }

test output:

java.lang.AssertionError: 
Expected: is "b,c,a,d\n2,3,1,4\n"
     but: was "b,c,a,d\n1,2,3,4\n"