Trim whitespaces for numeric fields

This pull request implements trimming of whitespaces for numeric fields as was suggested in #53 by me. It's implemented by changes to parse{Double,Signed,Unsigned}. Tests are included.

It leads to ~5% or so slowdown. Exact numbers fluctuate between benchmark runs

```
                                                       name  new/old
1           positional/decode/presidents/without conversion 1.0887817
2              positional/decode/presidents/with conversion 1.0261922
3 positional/decode/streaming/presidents/without conversion 0.9288449
4    positional/decode/streaming/presidents/with conversion 1.0274190
5              positional/encode/presidents/with conversion 0.9919386
6                named/decode/presidents/without conversion 1.0618138
7                   named/decode/presidents/with conversion 0.9508360
8                   named/encode/presidents/with conversion 0.9962220
9                                       comparison/lazy-csv 0.9730873  
```

P.S. below R script I used to generate table. Maybe it could be useful too
```
compare <- function(baseline,test) {
    q1 <- read.csv(baseline,stringsAsFactors = FALSE)
    q2 <- read.csv(test,    stringsAsFactors = FALSE)
    
    slowdown <- as.numeric(q2$Mean) / as.numeric( q1$Mean )
    plot(slowdown, col='red')
    abline(h=1)
    abline(h=mean(slowdown), col='red')
    #
    print( data.frame(name=q1$Name, slowdown=slowdown))
}
```